We plan to test the hypothesis that partial dysautonomia is the underlying pathophysiology for a significant portion of patients with orthostatic tachycardia. A secondary goal is to find effective treatment for this syndrome. To test this hypothesis will require establishing a number of postulates, which we will consider in the framework of specific aims. To establish this "partial dysautonomia" syndrome, it will be necessary to show that: (1) Central autonomic regulation is intact; (2) There is intact proximal autonomic innervation; (3) There is disrupted distal innervation (distal dysautonomia); (4) There is distal denervation hypersensitivity (in consequence of the disrupted innervation) in the face of absent or minimal proximal denervation hypersensitivity; and (5) Hypovolemia exists and correlates with the degree of dysautonomia. Finally in (6) we will test the acute effects of several potential treatment regimens on cardiovascular function.